1. Field of the Invention
The invention relates to an apparatus and method for riveting two or more sheets together using a self-piercing rivet.
2. Background Art
Self-piercing rivets are used in many industries because they provide a relatively simple one-step technique for joining metal sheets together. Presently, self-piercing rivet tools use a die with solid non-moveable parts to form a rivet that is used to join metal sheets. Self piercing rivets are secured in a cold forming process in which a semi-tubular rivet is pressed into overlapping metal sheets to mechanically fasten the metal sheets together. More specifically, the rivet pierces through an upper metal sheet and then embeds into a lower metal sheet to join the metal sheets. Self-piercing rivets do not require forming a hole in the metal sheets before riveting.
Manufacturers are adopting thinner and stronger materials to reduce the weight of manufactured products. The use of self-piercing rivets to fasten metal sheets made of stronger materials, such as high-strength low-alloy (HSLA) steel, creates challenges. One of the challenges to riveting stronger materials is that increased pressure is required to drive and deform the rivets. The pressure developed within the die cavity increases as the rivet is driven into the metal sheets. Increased pressure applied within the cavity during the riveting process causes increased resistance to penetration by the rivet. To overcome this resistance, the concept has been proposed to increase the strength of the cylindrical shaft of the rivet. Higher strength rivets are more expensive and more difficult to install. The increased pressure within the cavity also limits the types of self-piercing rivets that can be used in the self-piercing riveting process. In effect, increased pressure in the cavity substantially limits the range of materials that can be fastened together by self-piercing rivets.
The invention addresses one or more of the above problems and limitations as summarized below.